This invention relates to position sensing and in, particular, to apparatus and methods for effecting precise, non-contact, three-coordinate position sensing suitable, e.g., for use in robotic-type semiconductor wafer handling.
Semiconductor wafer processing equipment such as deposition reactors and etchers increasingly require automatic position sensing systems to locate targets such as wafer-holding susceptors and pedestals. Automatic position sensing is very useful, particularly where the targets may be moved or have unpredictable position changes, for example, as the result of high temperature cycling during the associated processing sequence. Typically such systems are used to determine the distance between the sensor and the sensed object at several points on the object by directing a beam of electromatic radiation at a near normal angle of incidence onto the sensed surface for reflection and detection by an associated detection circuit.
As an example, commonly assigned co-pending U.S. Ser. No. 693,722, entitled SEMICODUCTOR PROCESSING SYSTEM WITH ROBOTIC AUTO LOADER AND LOAD LOCK, filed Jan. 22, 1985, in the name of DAN MAYDAN ET AL, discloses a concurrently developed, state-of-the-art, non-contact, adaptive optical sensing system which is part of the robotic wafer handling system 10 shown schematically in FIG. 1. The Maydan et al application is incorporated herein by reference. This Maydan sensing system is used to detect and map the position of a wafer-mounting electrode or pedestal 11 relative to a robotic wafer gripper 12. In the system 10, a sensor 13 is used to determine the gripper 12-to-pedestal 11 distance at several points along the pedestal, under the control of computer 14. The computer is a VME (Versa Modular European) system controller which utilizes the position information to calculate the angular orientation and the position of the pedestal 11 relative to robot 16.
The automated wafer handling system or autoloader 10 automatically loads and unloads wafers 17 from the pedestal 11 in a processing system such as an ion-assisted plasma etching chamber. A plurality of gripping elements 18 are reciprocally movable in radial directions along transparent circular base 19 of the gripper 12 to cooperatively grip and release the wafers 17 for loading and unloading. The optical sensor 13 is mounted behind and/or to the side of the gripper base 19. The wafer gripper 12 is itself mounted on the robot 16 for (R, Z, .theta.) movement. That is, the gripper head 12 is mounted on robot arm 21 which is reciprocally movable along the R axis. The robot itself is reciprocally movable along Z and .theta. axes. R. Z and .theta. movement is effected by stepper motors (not shown) which are controlled by the system VME controller 14.
The wafer gripper sensor unit 13 transmits a focused IR beam at an angle of about 90.degree. (i.e., normal incidence) onto the pedestals 11 and detects the resultant reflected beam and transmits a resultant amplified, digitized electrical signal which contains information representing the distance between the sensor unit 13 and the pedestal 11. That is, the output signal from the sensor unit 13 is applied to amplfiier 22 and to analog/digital converter 23, then transmitted to the controller 14 for use in calculating the pedestal orientation and position and, ultimately, for use in controlling the three (R, .theta., Z) robot stepper motors.
One approach which takes advantage of the small focal spot size and sensitivity of sensor unit 13 involves determining the sensor 13-to-pedestal 11 distance at two points on the pedestal and applying triangulation techniques to calculate the precise angle of inclination and the associated wafer gripper travel distance for the pedestal.
In a preferred embodiment, the sensor unit 13 is the HP (Hewlett Packard) HEDS-1,000 High Resolution Optical Reflective Sensor. The optical spot (700 nm wavelength) is focused to a very small spot diameter of about 0.2 mm at 4.5 mm from the detector window. This detector was originally designed for bar code scanner readers. Because of its sensitivity to the actual distance (R axis distance), it has proven very useful for position sensing and determination.
However, it has been found that the very small focal spot size provided by the sensor 13, which advantageously permits precise determination of the gripper-to-pedestal distances, precludes use of the sensor in a parallel scanning mode, for example, to detect the location of articles such as pedestals on the wafers. The point focus of the sensor would cause the scanning sensor to constantly shift in and out of focus with the result that the electrical output would be obscured by noise.